describe: don't abort too early when searching tags

describe: don't abort too early when searching tags

When searching the commit graph for tag candidates, `git-describe`
will stop as soon as there is only one active branch left and
it already found an annotated tag as a candidate.

This works well as long as all branches eventually connect back
to a common root, but if the tags are found across branches
with no common ancestor

                  B
                  o----.
                        \
          o-----o---o----x
          A

it can happen that the search on one branch terminates prematurely
because a tag was found on another, independent branch. This scenario
isn't quite as obscure as it sounds, since cloning with a limited
depth often introduces many independent "dead ends" into the commit
graph.

The help text of `git-describe` states pretty clearly that when
describing a commit D, the number appended to the emitted tag X should
correspond to the number of commits found by `git log X..D`.

Thus, this commit modifies the stopping condition to only abort
the search when only one branch is left to search *and* all current
best candidates are descendants from that branch.

For repositories with a single root, this condition is always
true: When the search is reduced to a single active branch, the
current commit must be an ancestor of *all* tag candidates. This
means that in the common case, this change will have no negative
performance impact since the same number of commits as before will
be traversed.

Signed-off-by: Benno Evers <benno@bmevers.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>